Idle thoughts – Unstructured musings from Joel Dunn…

Daily Archives: April 30, 2018

Well, it’s actually only been 13 days, not two weeks, but close enough, since the installation started a couple days earlier. Our system was installed by Southern Energy Management, based in Morrisville, NC, who did an outstanding job. The system installed is composed of 15 REC TwinPeak 290 panels, 11 facing due south and 4 facing west, for a nominal capacity of 4.35 kW. The plan had been for all to face south, but the roof layout and potential for shading dictated the placement. With some of the panels facing west, the peak capacity is slightly less, but the “tail” of power generation is longer in the afternoon, due to the west-facing panels. These are connected to a 6000 W single-phase SolarEdge inverter. I asked for an inverter of higher capacity in case I ever wanted to add a few more panels.

Actually, the system isn’t complete yet, as we’ve got a Tesla Powerwall on order (on backorder, like everything else Tesla makes 😉 ) but telemetry from Southern Energy is that we’ll get it in August, so that’s at least on the roadmap. The Powerwall is an important part of the installation as the local utility does not offer “Net Metering.” This is the arrangement with your utility where you can sent your generated but unused power to the grid and you receive payment for each kWh that you send to the grid. So, we have to use all the power we generate, or it evaporates. That’s where the Powerwall comes in. It has a 13.5 kWh capacity, and absorbs power when you are not using it in the house. The solar system feeds the Powerwall, and the Powerwall then supplies current to the house at 5 kW continuous, 7 kW peak. It will supply power when the sun is down, or if a cloud passes over when a large load comes online. This should enable self-consumption of most of the power generated by the panels.

Currently, we’d be wasting a lot of potential power without being smart about when to run various appliances. For example, run the washing machine, dryer, and dishwasher between 10 and 4 (DST). Take showers mid-day as well, as the water heater is a big power draw. But the big thing is to intelligently manage charging of the electric car. It can hold 60 kWh of power, far more than can be generated, and can accept a charge at up to 7.2 kW. However, the JuiceBox Pro 40 EVSE we have for charging can set various charge rates below its 10 kW maximum, and by spreading the charge out during the sunny part of the day (when you don’t have to charge fast), you can capture much more of the solar power. Here’s a graph that shows this clearly. The JuiceBox is set to charge at a maximum of 3.6 kW.

The large “flat” peak is the car charge from about 0930 to 1530, for about 6 hours at 3.6 kW, or around 21-22 kW of demand. Note, however, that the system peaked at about 3.6 kW for only a short while at “noon” (1 PM DST). The demand from the car, combined with the baseline load of about .5 kW was more than the power that could be generated. The sharp peaks are the water heater, or at 0800 coffee and breakfast, and at 1730, cooking dinner. However, the solar still covered 54% of the house demand, and captured nearly 24 kWh. The Powerwall will work in the same way, capturing power left on the table to cover the overnight hours you can see on each side of the graph. We don’t charge the car every day, and this is where the automatic charge of the Powerwall will be very helpful. Without charging the car, usage that would be covered is more like 5-8 kWh. This, of course, is in a shoulder season without the heat pumps running, and we’ll see how that impacts overall consumption. The Powerwall, however, will ensure that we grab at least 20 kWh or so each day when the sun is shining.

Stay tuned for future analysis once we get the Powerwall in place. It’s really interesting to see how you use your electricity, and it can prompt changes in behavior.